Exhaust gas sensors in the form of lambda sensors are used in the exhaust gas channel of an internal combustion engine to determine the composition of an air-fuel mixture supplied to the internal combustion engine. According to statutory regulations, exhaust gas sensors must be tested for proper functioning during operation, i.e., for a so-called “measuring ability.” For this purpose, a gas mixture of a known composition may be supplied to the exhaust gas sensor, and the output signal of the exhaust gas sensor may be evaluated. During testing of a lambda sensor for proper function in the case of a “rich” air-fuel ratio having a lambda value of less than 1, and during testing of the so-called “rich measuring ability,” such a mixture would thus have to be supplied to the lambda sensor. In particular in the case of diesel engines, which are operated in the range of a leaner exhaust gas during normal operation, rich operation for a diagnosis results in increased emissions and additional fuel consumption and is therefore preferably avoided. Other possibilities for supplying a rich gas composition to the lambda sensor without intervening in engine operation have not been implementable for a variety of reasons.
However, a reliable diagnosis by another method is also subject to disadvantages. The pump current of the lambda sensor is used to determine the lambda value. The pump current is positive in the case of a lean exhaust gas and is negative in the case of a rich exhaust gas. The transport processes within the lambda sensor with lean exhaust gas and rich exhaust gas differ greatly with regard to the direction of the current, the electrode reaction and gases on the electrodes. For a realistic diagnosis, these conditions must therefore be provided inasmuch as is necessary for the diagnosis.
Exhaust gas sensors may be designed as broadband lambda sensors, lambda step change sensors, dual-cell broadband lambda sensors or single-cell limiting current sensors. Single-cell limiting current sensors may have the particular feature of containing a large reference gas channel for oxygen storage.
German Patent Application No. DE 102011005490 A1 describes a method for operating a sensor element for detecting at least one property of a gas in a measuring gas space, the sensor element having at least one pump cell, including at least two electrodes and at least one solid electrolyte connecting the electrodes, at least one first electrode of the pump cell being able to be acted upon with gas from the measuring gas space, at least one second electrode of the pump cell being connected to at least one reference channel, a check being carried out in the method to ascertain whether a pump current is limited by the pump cell by acting upon the first electrode with gas or whether the pump current is limited by the reference channel. This document does not describe either the diagnosis of the rich measuring ability or the possibility of creating rich gas components in the measuring gas space in a targeted manner for diagnosing the rich measuring ability of the lambda sensor.
German Patent Application No. DE 102010039188 A1 describes a method for detecting at least one property of a gas in a measuring gas space, in particular for detecting a gas component of the gas, at least one sensor element being used with at least one cell, the cell including at least one first electrode, at least one second electrode and at least one solid electrolyte connecting the first electrode and the second electrode, the first electrode being able to be acted upon with the gas from the measuring gas space, the second electrode being connected to at least one reference gas space, the reference gas space being configured to store a supply of the gas component of the gas, the method including at least two operating modes:                at least one measuring mode, the cell being operated as a pump cell in the measuring mode and the property being deduced from at least one pump current through the pump cell, and        at least one diagnostic mode, a storage capacity of the reference gas space being tested in the diagnostic mode, at least one measured variable influenced by a Nernst voltage applied to the cell being detected and the storage capacity being deduced from the measured variable.        
This document describes how the oxygen storage capacity within the lambda sensor may be influenced with the pump current. In addition, measuring modes are described, including details of the exhaust gas measurement in the case of engines operated in lean mode, in particular diesel engines. However, they do not discuss the diagnosis of the rich measuring ability.
German Patent Application No. DE 102010039392 A1 describes a device for detecting an oxygen component of a gas in a measuring gas space which includes at least one sensor element, the sensor element having at least one Nernst cell including at least one first electrode, at least one second electrode and at least one solid electrolyte connecting the first electrode and the second electrode, the first electrode being able to be acted upon by gas from the measuring gas space, the second electrode being situated in a reference gas space, the device also including at least one control, the control being configured to detect a voltage of the Nernst cell, the control also including an admission device for generating a reference pump current through the Nernst cell, the admission device being configured in such a way that a difference ΔIprel of the reference pump currents amounts to no more than 50% of reference pump current Iprel averaged over the air ratio ranges in the rich air ratio range and in the lean air ratio range. This document also describes the wiring of the exhaust gas sensor but without any description of or solution to the problems of diagnosing the rich measuring ability of engines being operated in lean mode.